Fingerprints are invariable, unique and convenient, which thus provide a high-level identity security authentication. During the identity security authentication, fingerprint sensors are generally used as devices for automatically acquiring the fingerprints.
Based on the sensing mechanism, fingerprint sensors are mainly categorized into optical fingerprint sensors, semiconductor capacitive sensors, semiconductor thermistor sensors, semiconductor pressure sensitive sensors, ultrasound sensors, radio frequency (RF) sensors and the like.
Using a semiconductor capacitive sensor as an example, a finger touching on a “flat plate” in which thousands of semiconductor devices are integrated, forms a side of a capacitor. Since the fingerprints have projections and depressions on the surface thereof, the projections are ridges and the depressions are valleys, an actual distance from a projection to the flat plate is different from an actual distance from a depression to the flat plate, a capacitance formed by the projection and flat plate is also different from that formed by the depression and the flat plate. The acquired different capacitance values are collected, such that fingerprint acquisition is completion.
However, during fingerprint acquisition, due to a parasitic capacitor in the fingerprint sensor or a base capacitor in the fingerprint sensor, a great base signal may be produced at an integrator. The base signal is at least 100 times of a valid signal, and upon amplification, the base signal is much greater than the valid signal; thus, the valid signal is relatively smaller. In addition, the amplified base signal may cause the integrator to reach a saturated state, and thus an output dynamic range of the integrator is small.